1. Field of the Invention
The present invention relates to the measurement of power in a system consisting of audio power amplifier(s) (or simply, amplifier) and speaker(s), and a method of protecting the speaker(s) from power overload and damage, and possible consequential damage to the amplifier.
2. Description of the Prior Art
The present invention, an audio power meter (APM), is intended to measure and display the amount of power flowing from an audio amplifier to a speaker (or speakers in all that follows). The APM also acts to protect the speaker from overload and failure, and possible consequential damage to the amplifier when driving a failed speaker. There is a need for an inexpensive instrument that can be used by working musicians to gauge the amount of power they use in the practice of their art, and consequently the amount of sound they are producing. This is done in an effort to 1) provide consistent performances, and 2) protect the musician's equipment investment by preventing equipment damage. These goals are important when surveying the prior art.
As background, the user of the APM is typically a non-technical musician, unschooled in engineering. The user understands basically how to connect a speaker cabinet to an amplifier, but does not understand the concepts of voltage, current, or impedance. The user does understand the concept of power in the vague sense that more power equals louder sound, and that too much power damages speakers. The user is however acutely aware of the importance of equipment reliability, the tone the equipment lends to his unique artistic sound, the consistency of performance that his equipment enables, and the monetary and collector's value of his (perhaps antique) amplifiers and speakers. These factors also must be considered in the survey of the prior art.
Tokatian (U.S. Pat. No. 6,201,680, which issued on Mar. 13, 2001) teaches a speaker protection circuit which attaches to the signal path between the amplifier and speaker. The method promises fast transient response and rapid protection of the speaker. However, several disadvantages are apparent. The threshold of protection is a function of the impedance of the speaker, and if the user attaches a speaker of a different impedance (which is likely because musicians swap and trade equipment frequently), the protection may be compromised. The circuit judges a transient based on its voltage amplitude, which is of course load impedance dependent. This is a problem because certain voltage appearing across the terminals of an 8 ohm speaker represents a lower power than the same voltage appearing across the terminals of a 4 ohm speaker. Thus the protection of two speakers with identical power ratings is a function of the speaker impedance, which can be changed at any time by the user through replacement of the speaker. The relay contacts in the invention must potentially switch large voltages and currents in order to protect the speaker, requiring an expensive relay to avoid contact burn or sticking. The relay is the least reliable component in the taught invention, and should be avoided for maximum reliability. Additionally, for musical instrument application, the occurrence of fast transient signals is common, as produced by the musician, and a transient suppressor that operates as quickly as the invention's (65 nanoseconds) may cause audible and annoying side effects. Longer transients (6 milliseconds-14 milliseconds) that cause disconnection of the speaker by the invention are common in live musical performances.
GrosJean (U.S. Pat. No. 3,959,735, which issued on May 25, 1976) teaches a method whereby speaker overloads are prevented by disconnecting the power supply to the power amplifier in the case of overload. In the case of musical instrument amplifiers with which the APM is intended to be used, the power switching would have to be performed on the AC line input to the power amplifier in order to be generally applicable to many makes and models of amplifiers. This would require a significant power switching arrangement, either using large relays or power semiconductors. That in turn would require more stringent safety approvals and testing, and consequent increased cost. Additionally, restoring the audio power amplifier to working order after the overload has subsided may take several seconds due to delays in modern amplifier power supplies, or delays caused by warm-up in vacuum tube amplifiers. These delays are disruptive to musical performances.
Botti et al. (U.S. Pat. No. 5,315,268, which issued on May 24, 1994) teach a method of amplifier gain modification which causes a gain reduction in an upstream amplifier stage in the case that the output exceeds some preset reference level, this to avoid the engagement of thermal overload circuits in the amplifier which may cause distortion. While this method may ensure that the audio signal coming out of the amplifier is substantially distortion free at most temperatures (though at lower amplitudes), it does nothing to protect the speaker connected to the amplifier. Add to that the fact that many musicians actually seek to obtain distortion from their amplifiers, and the Botti et al. method is seen to be unusable in that application. Also, any amplifier or speaker protection mechanism that is temperature dependent is of less value because speaker protection must be performed at cold ambient temperatures as well as hot, as are found in outdoor performances. Further, the active electronic variable gain cell taught introduces noise into the signal path.
Ikoma (U.S. Pat. No. 4,581,589, which issued on Apr. 8, 1986) also teaches a clipping prevention technique that implements an upstream attenuation method. However, like the Botti et al. method, this invention seeks to prevent clipping and distortion. Once again many musicians actually seek to obtain distortion from their amplifiers, so the clipping criteria cannot be used to gauge amplifier performance or speaker protection. Protection of the amplifier or speaker must be judged using a more appropriate criteria, such as power moving to the load. Also, Ikoma teaches the use of a light dependent resistor in the attenuator stage. Such devices are highly nonlinear and introduce harmonic distortion of their own on the order of one to several percent when attenuating the audio signal, which is counterproductive to the application as intended. Such devices also consume significant power when used in battery operated equipment.
Fink (U.S. Pat. No. 5,719,526, which issued on Feb. 17, 1998) teaches a load monitoring method that computes power delivered to a load. This invention operates to monitor a load, but not to protect it. It also modifies the power amplifier transfer function internally (literally, “sends the control signals to the power amplifier”) based on a calculation of the load impedance or power. The Fink patent specifies that the invention is to be applied “within” the amplifier chassis. Applying this technique to existing amplifiers would be impractical and would require internal modifications. For use with existing amplifiers, it would be better to retain such computed information within the power measurement device, and adjust the amplifier transfer function (specifically, gain) externally, negating the requirement to communicate with or modify the power amplifier at all. The advantage of the APM of the present invention over the Fink method is that no internal modifications are made to the amplifier. This is important when using the power meter and protection features with valuable antique amplifiers that would be reduced in value if modified in the least, even cosmetically.
Haigler (U.S. Pat. No. 4,887,298, which issued on Dec. 12, 1989) teaches a method of speaker protection that operates in more expensive installations (professional sound reinforcement systems) that include a speaker sense line. If the sense line fails (is disconnected for example), then the amplifier may overdrive the speaker. Haigler has invented a protection for a protective circuit, the protective circuit being the sense line. The APM of the present invention does not require or use a sense line, and is fully functional to protect a speaker without it. The Haigler invention waits a predetermined amount of time and re-enables full power to the load, and thus oscillates when the speaker sense line fails, causing an “aural indication that a failure on the sense line has been detected.” This behavior is highly undesirable in musical performance settings. The APM of the present invention attenuates the audio until directed otherwise by the user, preventing distraction during a performance. This also prevents large signals from repetitively overdriving the loudspeaker.
Dorrough (U.S. Pat. No. 5,751,819, which issued on May 12, 1998) teaches a method of implementation of a level meter for display of digital audio streams. However, the invention uses average and peak voltage level measurements which are inferior to power measurements in the speaker protection application, since speakers are customarily rated not in peak or average voltage terms, but watts.
Neely et al. (U.S. Pat. No. 5,327,101, which issued on Jul. 5, 1994) teach a method of clipping reduction in an inverting operational amplifier. This method is inapplicable in external connection to existing audio power amplifiers and speakers such as used by musicians.
Klippel (U.S. Pat. No. 5,528,695, which issued on Jun. 18, 1996) teach a protection method for speakers which depends on the peak signal applied to the speaker. While this may provide some protection, it is impractical in application to existing musical instrument speakers because they are customarily not rated in terms of peak voltage, current, or power. The user has no idea of how to set the protection threshold using the Klippel invention. The APM of the present invention measures power and controls its audio attenuator using that result, and the user sets the protection threshold directly in terms of watts.
The above survey of prior art reveals many inventions that seek to protect amplifiers and speakers, but which are not suitable for use by the performing musician. The APM of the present invention is, however, designed exactly for that situation.